1. Technical Field
The present invention relates to hold circuits for use in subscribers electronic telephones, and more particularly, to electronic telephone hold circuits which require extremely reliable operation under long loop conditions.
2. Background Art
Since the development of circuitry employing active components, i.e., solid state devices, etc., many additional features have been very easily added to conventional telephone circuitry. One such feature has been the addition of the so-called "hold" circuitry. With the provision of such a circuit, a telephone call may be electronically latched onto an artificial load allowing the handset to be returned to the hookswitch, until the call is picked up on an associated extension telephone or again picked up at the original phone, at which time the hold condition is dropped automatically.
Many previous hold circuits frequently made use of expensive components such as latching relays and/or integrated circuit voltage comparators and related circuitry adding substantial cost to the telephone. Sometimes such hold circuits had poor sensitivity because of the compromise between trigger "on" sensitivity and the ability to drop completely on high resistance extensions and/or in the presence of low central office battery voltage. Some hold circuit methods required the use of a microprocessor and related circuitry to sense the line voltage change with conversion of the result to a digital signal. It is obvious that all such circuitry suffers to some extent from a great degree of over complexity and attendant thereon a reduction in the reliability, stemming from the increased number of components required in such circuit design.
One particular problem of reliability existing in the previous hold circuitry design is the possibility of dropping the call inadvertently by a slight bounce of the hookswitch contacts when going on-hook after a hold condition has been initiated. This premature hold drop was more likely to occur on long loops with low central office battery voltages or with low DC resistance telephones. An associated problem has also been the low level of brightness of on associated light emitting diode or indicator lamp used to indicate the hold condition, particularly under long loop conditions with the handset off-hook. Such low level conditions traditionally force the indicator lamp to be mounted on the top surface of the telephone so that it might be more readily seen. This method which is quite expensive instead of being able to mount the lamp or light emitting diode on the included printed circuit cord and then utilizing fiber optics or "light pipes" to conduct light to the top surface of the telephone for viewing.
Accordingly, it is the object of the present invention to provide a new and useful telephone hold circuit that provides a greater degree of reliability of operation, fewer components and higher brightness of indicator illumination on long telephone loop conditions.